What's that word? Does it mean "circular reasoning"?

As to the first part of your question about finding a word you've forgotten, you could try a reverse dictionary.

I also love, love, love my dog-eared copy of Roget's Thesaurus. (I should probably have looked up some synonyms for "love.") There is something to be said for thumbing through it and finding a category of words to ponder. Serendipity at its best, I tell you. There's an online version of Roget's too, but it's just not the same.

Circular argument and petitio principii are other terms I've seen for this. Wikipedia says it's also called begging the question:

Begging the question (Latin petitio principii, "assuming the initial point") is a type of logical fallacy in which a proposition is made that that uses its own premise as proof of the proposition. In other words, it is a statement that refers to its own assertion to prove the assertion.

What about tautological ?

This may convey what you require.

A fallacy is incorrect reasoning, of which circular logic is one kind. Wikipedia also has a List of Fallacies which may help you.